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Re: [ietf-smtp] IETF Policy on dogfood consumption or avoidance - SMTP version

2019-12-20 03:30:16
On 19/12/2019 22:52, Hector Santos wrote:

The fact is, there is a SMTP protocol rule of using a HELO/EHLO [ip-literal] that MUST match the connection IP.  If not, it is a strong reason for rejection.  I have wcSAP stats for that.

The thing is that RFC 5321 says you MUST NOT reject mail if the EHLO parameter isn't "right".

"An SMTP server MAY verify that the domain name argument in the EHLO
   command actually corresponds to the IP address of the client.
   However, if the verification fails, the server MUST NOT refuse to
   accept a message on that basis."

For the 'address literal' option, all it says is "an address literal SHOULD be substituted for the    domain name" (note the word 'an' - no mention is made (AFAICS) of any correctness requirement).

Note that I'm not arguing against tests which require that, just that saying that the address literal MUST match the connection IP is going beyond the standards (IMHO).

It's actually quite hard to automatically get a correct address literal if you're behind NAT, and if you're also on a dynamic IP, then it's approaching the impossible. Now, you may say that 'if the sender is on a dynamic IP we don't want to receive mail from them', and that's a valid viewpoint, but I'm not sure the SMTP standard should prohibit SMTP senders being on dynamic IP addresses.


Paul Smith Computer Services
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